It is a moment that speaks volumes. While the Kfir combat jets of the Sri Lankan airforce scream overhead and the heavy artillery of the Sri Lankan army maintains a remorseless barrage on the ground below, a family of terrified Tamil civilians huddle in a shallow trench.

It is January 2009, and the beginning of the end of the 25-year war for an independent state of Tamil Eelam. The increasingly battered remnants of the secessionist Liberation Tigers of Tamil Eelam are on the run and along with perhaps 400,000 Tamil civilians, they are being herded into an ever smaller area of land in north-east Sri Lanka.

The brutal Sinhalese government of President Mahinda Rajapaksa – armed with the rhetoric of the "international war on terror" and the tacit support of most of the world – believes it has a licence to eradicate the Tamil Tigers for ever. The UN and other international agencies have been pressurised into leaving, the world's media have been excluded and critical local journalists have been exiled, disappeared or murdered. The world is looking away.

As the shells fall, the trench provides little protection. It is only three feet deep and the adults, crouched protectively over their children, can barely get their heads below the level of the ground. But someone has not jumped into the trench; someone with a small video camera. Despite the nearby crump of the shells, he keeps filming. A woman in the trench is clutching a baby and crying desperately. "Please get in the bunker! Don't take the video!" she shouts in Tamil. "What are you going to do with the video? They are killing everyone …"

Over the next four months, two things happen. First the government does appear – through the deliberate targeting of hospitals and laughably named "no-fire zones" – to try to kill as many Tamils as possible; perhaps 40,000 civilians, perhaps far more (that they were in effect aided and abetted by the Tamil Tigers who used those same civilians as human shields does not in any way lessen the government's culpability.)

The other thing that happens is that the cameraman – or camerawoman – keep filming. As do many others. Sometimes on small domestic cameras, sometimes on phones. The Tamil side gets many of these images out on the internet. Other footage – grotesque images of war crimes, execution and brutality by Sri Lankan armed forces – is recorded by the perpetrators themselves on mobile phones. And so two years later there is an answer to that woman's terrified question: "What are you going to do with the video?"

We have been able to make that footage – along with many hours of even more disturbing images – into a film which might, belatedly, play a part in bringing the perpetrators of those crimes to justice. It is called Sri Lanka's Killing Fields and will be aired on Tuesday night on Channel 4.

The film has been creating a stir partly because some of the images are probably the most horrific ever to have been broadcast on mainstream television (hence the late-night slot). But we hope it will be remembered for another reason: we hope it will act as a reminder to those who would massacre their own people – and perhaps more importantly to the UN, the international community and the world's powers – that modern technology means you will never again get away with committing these kind of war crimes and crimes against humanity in secret. From now on, the victims – and all too often the perpetrators as well – will keep a record.

But that is only the first stage. The next is to ensure that this awful evidence is not ignored. These pictures push to the limit every normal rule of what is acceptable on television. You will see prisoners, bound and gagged, being executed in cold blood. You will see innocent civilians dying in agony on the ground in makeshift hospitals, which have been denied medicines and supplies by the Sri Lankan government. But if this is the only way to make people take this seriously, we believe it is the right thing to show these images.

Two months ago the UN's "panel of experts" concluded that the allegations of war crimes and crimes against humanity by both the Sri Lankan government and the Tamil Tigers are "credible", and called on Ban Ki Moon to create an international mechanism to properly investigate them. So far he has declined – saying he doesn't have the authority. That is debatable, but the nation states of the UN through bodies such as the security council and the human rights council do have that authority. If the UN fails yet again, the message to every tyrant and repressive government will be clear: if you want to kill your own people with impunity, you will probably get away with it.